” Is there a limit to how many infantry units you can upgrade to motorized? I assume there isn’t since it’s not stated.”
No limit. You may spend all your IPP on it having 1 minor factory with a supply path to the regular infantries. Such are the current rules (may 2021)
Optional house rule: count motorized upgrades as using up production slots for factories.

“If Russia and Germany sign the pact, can the Germans use the northern sea route to trace a lend-lease supply path? My hunch is they can’t, but I’m still curious”
No, because it is not mentioned specifically or generally.
Optional house rule: Let the USSR player decide on permission.